Hard tissue anchors and delivery devices

ABSTRACT

The present invention provides devices, systems and methods for anchoring medical devices to hard tissues, such as bones or bony structures, particularly vertebrae. By anchoring these medical devices directly to the surrounding hard tissue, the devices are anchored closer to the source of treatment. This provides additional stability and reduces migration of the device at the treatment site. Also, by attaching to hard tissue rather than soft tissue, a stronger attachment is often able to be made.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/175,488, filed Jul. 1, 2011, Publication No. US 2011-0257693-A1, which is a continuation of U.S. patent application Ser. No. 11/952,081, filed on Dec. 6, 2007, Publication No. US-2008-0183221-A1, now abandoned, which claims priority of U.S. Provisional Patent Application No. 60/873,549, filed on Dec. 6, 2006, each of which is herein incorporated by reference in its entirety.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not Applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK

Not Applicable

BACKGROUND OF THE INVENTION

A variety of implantable medical devices are used to treat portions of the anatomy which reside near bones or bony structures within the body of a patient. Such devices are typically anchored in place by suturing portions of the device to surrounding soft tissue. Often the device includes suture holes designed specifically for this purpose at predetermined locations along the device. Thus, the device may only be sutured at these locations, limiting the areas and types of tissue available for suturing thereto. Often, the location is far from the treatment site. Such distance and instability of anchoring tissue can contribute to lead migration and pull-out.

For example, conventional spinal cord stimulators (SCS) are positioned along the spinal column to treat pain. A conventional SCS system comprises an implantable lead and an implantable power source or implantable pulse pulse generator IPG. Using fluoroscopy, the lead is implanted into the epidural space of the spinal column and positioned against the dura layer of the spinal cord. The lead extends from the spinal column to the IPG which is remotely implanted. Typically, the lead is sutured to soft tissue remote from the point of entry into the epidural space. And, lead migration and pull-out are common problems associated with SCS.

Therefore, it is desired to provide a more stable anchoring system for implantable devices, such as leads. Such an anchoring system should provide anchoring at desired locations rather than merely at locations along the device which are predesigned for anchoring. Such anchoring should also assist in resisting migration and pull-out. At least some of these objectives will be met by the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention provides devices, systems and methods for anchoring medical devices to hard tissues, such as bones or bony structures, particularly vertebrae. A variety of medical devices are used to treat portions of the anatomy which reside near bones or bony structures within the body of a patient. The devices and systems of the present invention are suitable for use with many of such medical devices and specialized devices used for particular treatments. By anchoring these directly to the surrounding hard tissue, the devices are anchored closer to the source of treatment. This provides additional stability and reduces migration of the device at the treatment site. Also, by attaching to hard tissue rather than soft tissue, a stronger attachment is often able to be made.

In a first aspect of the present invention, a hard tissue anchor is provided for securing an element to a hard tissue. In some embodiments, the hard tissue comprises a penetrating end shaped for penetrating the hard tissue, and a head having an aperture, wherein the aperture is configured to receive the element therethrough and wherein the head is configured to secure the element within the aperture. Typically, the element comprises a lead, however catheters or other devices may be used.

In some embodiments, the head includes a channel connected to the aperture, wherein the channel is configured allow passage of the element from outside of the head to the aperture. In some instances, the head is adjustable to close the channel, such as by deformation of the head. Optionally, deformation of the head may secure the element within the aperture. In some embodiments, the head further comprises a grommet disposed within the aperture. The grommet may assist in holding the element within the aperture.

In some embodiments, the penetrating end has a tapered, conical, notched, barbed or serrated shape. In such instances, the hard tissue anchor is considered a tack and is pressed into the hard tissue. In other embodiments, the penetrating end has a shank with a helical thread. In these instances, the hard tissue anchor is considered a screw and is rotated into the hard tissue.

In a second aspect of the present invention, a method is provided for anchoring an element to a hard tissue in a body: In some embodiments, the method comprises advancing a hard tissue anchor toward the hard tissue, wherein the anchor has a penetrating end and a head having an aperture, positioning the element within the aperture, and applying pressure to the head so as to drive the penetrating end at least partially into the hard tissue.

In some embodiments, applying pressure comprises applying pressure to the head so as to secure the element within the aperture. Optionally, applying pressure comprises deforming the head so as to secure the element within the aperture due to friction.

In some instances, the method further comprises implanting the element in the body. Such implanting may occur before the positioning step of positioning the element within the aperture. This allows the hard tissue anchors to be utilized with existing implanted systems.

In still further embodiments, the anchor includes a channel connected to the aperture and the method further comprises passing a portion of the element through the channel to the aperture. Optionally, applying pressure comprises deforming the head so as to at least partially close the channel.

To deliver a hard tissue anchor of the present invention, such methods may include mounting the head of the anchor on a distal end of an applicator. In some situations, advancing the hard tissue anchor toward the hard tissue comprises advancing the distal end of the applicator through a percutaneous access opening. In such instances, the applicator has a low profile suitable for such percutaneous delivery.

In some embodiments, applying pressure to the head comprises applying pressure to the applicator. Optionally, applying pressure to the applicator may comprise deforming the head by force of the applicator so as to secure the element within the aperture due to friction.

In a third aspect of the present invention, an applicator is provided for delivering a hard tissue anchor. In some embodiments, the applicator comprises an elongate body having a proximal end and a distal end, wherein the distal end is configured to receive a head of the hard tissue anchor, and a handle attached to the proximal end of the elongate body so that force applied to the handle is translatable to the head of the hard tissue anchor. Optionally, the elongate body may be shaped for passage through a percutaneous access opening.

In some embodiments, the applicator further comprises a release button for releasing the hard tissue anchor from the distal end of the elongate body. The distal end may include a recess for receiving the head, from which the hard tissue anchor is releasable.

When the hard tissue anchor comprises a bone tack, the force typically comprises longitudinal force which is translatable to a downward force on the head of the hard tissue anchor. When the hard tissue anchor comprises a bone screw, the force typically comprises rotational force which is translatable to rotation of the head of the hard tissue anchor. In such instances, the distal end may comprise a rotatable member joinable with the head, wherein the rotation force rotates the rotatable member.

Other objects and advantages of the present invention will become apparent from the detailed description to follow, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a hard tissue anchor of the present invention used with a conventional SCS system.

FIG. 2 illustrates a hard tissue anchor of the present invention used with a lead which is implanted near a DRG to provide selective stimulation thereto.

FIG. 3 illustrates an embodiment of a bone tack of the present invention.

FIG. 4 illustrates an embodiment of a bone tack having an element threaded through its head prior to implantation of the element.

FIG. 5 illustrates a side view of a tack having a channel along the top of the head.

FIG. 6 illustrates a top view of such the tack of FIG. 5.

FIG. 7 illustrates passing an element through a channel in the head of a bone tack.

FIG. 8 illustrates a lead surrounded by a silicone tube positioned within arms of the head of a bone tack.

FIGS. 9A-9B illustrate an embodiment of a bone tack having a grommet.

FIG. 10 illustrates a tack having a grommet wherein the channel of the grommet has been closed by crimping of the head.

FIGS. 11A, 11B, 11C, 11D illustrate front, side, top and bottom views, respectively, of one embodiment of a bone tack of the present invention.

FIG. 12 illustrates an applicator for delivery of a bone tack to a portion of a hard tissue.

FIG. 13 illustrates a distal end of the applicator having a recess for receiving a head of a bone tack.

FIG. 14 illustrates a bone tack securely fixed to an applicator during insertion via friction fit with a grommet.

FIGS. 15A, 15B, 15C, 15D, 15E illustrates various views elongate body of an applicator having an insert positionable within its distal end.

FIG. 16 illustrates an embodiment of a bone screw of the present invention.

FIGS. 17A, 17B, 17C, 17D, 17E illustrate various view of an embodiment of a bone screw.

FIGS. 18A-18B illustrate an applicator for delivery of a bone screw to a portion of a hard tissue.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides devices, systems and methods for anchoring medical devices to hard tissues, such as bones or bony structures, particularly vertebrae. A variety of medical devices are used to treat portions of the anatomy which reside near bones or bony structures within the body of a patient. For example, spinal cord stimulators (SCS) are positioned along the spinal column to treat pain. FIG. 1 illustrates a conventional SCS system comprising an implantable lead 100 and an implantable power source or implantable pulse pulse generator IPG. Using fluoroscopy, the lead 100 is implanted into the epidural space E of the spinal column S and positioned against the dura layer of the spinal cord. The lead 100 is implanted either through the skin via an epidural needle (for percutaneous leads) or directly and surgically through a mini laminotomy operation (for paddle leads). In either case, the leads 100 extend from the spinal column S to the IPG which is remotely implanted. Typically, the leads 100 are sutured to soft tissue remote from the point of entry into the epidural space E. Such suturing is often insufficient to adequately the implanted lead 100, thus leading to migration or pull-out. FIG. 1 illustrates a hard tissue anchor 600 of the present invention used in conjunction with the conventional SCS system to anchor the implantable lead 100. As shown, the anchor 600 can be used to attach the lead 100 a hard tissue, such as a vertebrae V near the point of entry to the epidural space E. This provides more secure anchoring by fixing to a harder tissue and reduces the distance between the distal portion of the lead and the site of anchoring. This assists in reducing migration and pull-out of the lead 100.

In addition, the devices, systems and methods of the present invention may be used to anchor other types of medical devices, in particular various other types of leads used to selectively stimulate the spinal anatomy, particularly the dorsal root or dorsal root ganglion (DRG). FIG. 2 illustrates a lead 150 which is implanted near a DRG to provide selective stimulation thereto. Examples of such leads are provided in U.S. patent application Ser. No. 11/952,049, filed Dec. 6, 2007, entitled “Grouped Leads For Spinal Stimulation,” Publication No. US-2008-0147156-A1 (Attorney Docket No. 10088-706.201/Client Ref No. SM-00610US) and U.S. patent application Ser. No. 11/952,053, filed Dec. 6, 2007, entitled “Grouped Leads For Posterior Access Of Directed Spinal Stimulation,” Publication No. US-2008-0140169-A1 (Attorney Docket No. 10088-707.201/Client Ref No. SM-00710US), both incorporated herein by reference. As shown, a hard tissue anchor 600 of the present invention may be used to anchor the lead 150 to a portion of the vertebrae V which is near the DRG. This anchors the lead 150 close to the stimulation site and reduces migration or pull-out of the lead 150.

The hard tissue anchors 600 of the present invention include bone tacks and bone screws. FIG. 3 illustrates an embodiment of a bone tack 601 of the present invention. The bone tack 601 can be used to anchor an element, such as a lead or catheter, to a bone or bony structure, such as near to a site of an intended application. In this embodiment, the bone tack 601 has a head 602 and a penetrating end 604 opposite the head 602. The penetrating end 604 may have a tapered, conical, notched, barbed, serrated or otherwise shaped end which is suitable for penetrating bone B, as shown. The head 602 includes an aperture 607 through which the element 152 can be threaded prior to implantation of the element 152, as illustrated in FIG. 4. The bone tack 601 is advanced along the element 152 to the desired anchoring position. Force is then applied to the head 602 to advance the penetrating end 604 into the bone B, thereby fixing the element 152 to the bone B at that location. This may be achieved during the implantation procedure of the element 150.

Other embodiments of the bone tack 601 are particularly suited for anchoring the element 150 at an anchoring location when it is less desirable to pre-load the anchor on the element 150. This may be the case when the element 150 is already implanted or it is not possible to advance an anchor over the element 150, such as from one of the ends of the element 150 to the anchoring location. In some of these embodiments, the head 602 of the bone tack 601 includes a channel 608 which connects to the aperture 607. FIG. 5 illustrates a side view of an embodiment of a tack 601 having such a channel 608 along the top of the head 602, and FIG. 6 illustrates a top view of such a tack 601. The tack 601 can be slipped over the element 150 through the channel 608 in the head 602 so that the element 150 passes through the aperture 607, as illustrated in FIG. 7. Thus, the tack 601 can be positioned at any location along the element 150. The channel 608 can then be closed by deformation of the head 602. Further deformation of the head 602 crimps the head 602 onto the element 150 resisting relative motion.

FIG. 8 illustrates a lead 610 surrounded by a silicone tube 612 positioned within arms 614 of the head 602 of a bone tack 601. Deforming or crimping of the head 602 (at least one arm 614) holds the silicone tube 612 in relation to the head 602 and further crimping holds the lead 610 in relation to the silicone tube 612. Thus, various degrees of deformation may be used to provide differing desired results.

In some embodiments, the tack 601 includes a grommet 606, as illustrated in FIGS. 9A-9B. The grommet 606 includes a channel which is alignable with the channel 608 of the head 602. Thus, an element 150 may be passed through the channel 608 of the head 602 and the aligned channel of the grommet 606. The grommet 606 assists in applying friction to the element 150 and protects the element 150 from possible damage. Deformation or crimping of the head 602 applies further friction to the element 150, such as fixing the element 150 within the grommet 606. FIG. 10 illustrates a tack 601 having a grommet 606 wherein the channel of the grommet 606 has been closed by crimping of the head 602. This illustrates the reduction in size of the aperture an therefore increased friction against the element 150.

FIGS. 11A, 11B, 11C, 11D provide front, side, top and bottom views, respectively, of one embodiment of a bone tack 601 of the present invention. In the front view illustrated in FIG. 11A, tack 601 includes head 602 and penetrating end 604. As shown in FIG. 11A, head 602 can be shaped like an incomplete ring to include aperture 607 and channel 608. Head 602 can have a 0.060 inch diameter, aperture 607 can have a 0.040 inch diameter, and channel 608 can be 0.008 inches wide, for example. The penetrating end 604 can have serrations which taper from a first serration 611 to a second serration 613 to a point 615. For example, first serration 611 can have a width of approximately 0.045 inches and extend outwards at an angle of approximately 50 degrees, second serration can have a width of approximately 0.035 inches and extend outwards at an angle of approximately 50 degrees, and penetrating end 604 can have a length of approximately 0.105 inches from the center of aperture 607 to point 615. Additionally, the distance from the center of aperture 607 to neck 609 can be approximately 0.029 inches, the distance from the center of aperture 607 to first serration 611 can be approximately 0.043 inches, and the distance from the center of aperture 607 to second serration 613 can be approximately 0.068 inches. Neck 609 of the penetrating end 604 can attach to head 602, and can extend outwards at an angle of approximately 30 degrees. FIG. 11B shows a side view of bone tack 601, including head 602 and penetrating end 604 having first serration 611, second serration 613, and point 615. As shown in FIG. 11B, penetrating end 604 can come to a sharp point 615 at an angle of approximately 40 degrees. Furthermore, the distance from second serration 613 to point 615 can be approximately 0.034 inches, for example. FIG. 11C shows a top view of bone tack 601. Head 602 can have a depth of approximately 0.009 inches, and penetrating end 604 can have a depth of approximately 0.025 inches, for example. FIG. 11D illustrates point 615 of penetrating end 604. Thus, the bone tacks 601 of the present invention typically have a small size to allow positioning in confined or hard to reach areas of the anatomy. It may be appreciated that such dimensions are exemplary and are not intended to limit the scope of the present invention.

The head 602 and a penetrating end 604 are typically formed from the same material and may comprise any biocompatible and/or bioresorbable material including but not limited to cobalt chromium, cobalt chromium alloys, titanium, titanium alloys, stainless steel, resorbable PGA or PLA, and PEEK.

The grommet 606 may be comprised of any soft biocompatible and/or bioresorbable material including but not limited to silicone or polyurethane. The grommet 606 could be an assembly or molded onto the tack 606.

The bone tacks 601 of the present invention are driven into a portion of bone B by mechanical force, such as tapping or pressing. Referring to FIG. 12, an applicator 620 is provided for delivery of the bone tack 601 to a portion of a bone B. The applicator 620 is designed so that the tack 601 can be delivered through a percutaneous access opening and positioned at an anchoring location via fluoroscopy or other imaging techniques. Typically, the applicator 620 comprises an elongate body 300 with a low profile to assist in accessing a variety of target locations within the body. The elongate body 300 has a proximal end 302 and a distal end 304, wherein the distal end 304 is configured to receive the hard tissue anchor 600. In most embodiments, the applicator 620 also includes a handle 306 attached to the proximal end 302 of the elongate body 300.

FIG. 13 illustrates an embodiment of a distal end 304 of the applicator 620 having a recess 624 for receiving a head 602 of a bone tack 601. In some embodiments, the bone tack 601 is securely fixed to the applicator 620 during insertion via friction fit with the grommet 606, as illustrated in FIG. 14. The tack 601 is penetrated and anchored into the bone B via the penetrating end 604, by application of downward or longitudinal force on the tack 601 by the applicator 620. Thus, force applied to the handle 306 is translatable to the head 602 of the hard tissue anchor 600 and drives the anchor 600 into the hard tissue. In some embodiments, such force also then crimps the head 602 onto an element passing through the aperture 607. The tack 601 can then be released from the applicator 620, such as with the use of a release button 626. The tack 601 is then left behind with the element passing therethrough.

In some embodiments, the distal end 304 is comprised of an insert that is inserted into the elongate body 300. FIGS. 15A-15E illustrate various views of an elongate body 300 having an insert 301. Typically the insert 301 is formed or machined so that together the insert 301 and the elongate body 300 desirably receive the bone tack 601. FIG. 15A illustrates a side view of an insert 301 having a recess 624 for receiving a bone tack 601. Here the recess 624 has a depth of 0.050 inches and a width of 0.060 inches. FIG. 15B illustrates an embodiment of an elongate body 300 having a length of 0.105 inches and a width of 0.28 inches. FIG. 15C illustrates a bottom view of an insert 301 showing recess 624. The insert 301 is inserted into a slot 303 in the elongate body 300, illustrated in FIG. 15D. In this embodiment, the slot 303 has a depth of 0.105 inches and a width of 0.028 inches. FIG. 15E illustrates a side view of the elongate body 300 having a notch 305. When a bone tack 601 is inserted into the distal end 304, as illustrated in FIG. 14, the aperture 607 of the bone tack 601 is exposed to allow an element to pass therethrough. Referring back to FIG. 15E, in this embodiment, the notch 305 has a width of 0.033 inches. It may be appreciated that the dimensions noted herein are examples.

Example methods of installing a bone tack 601 of the present invention are described herein. In one embodiment, a tack 601 of the present invention is mounted in an applicator 620 as described above. An element, such as a lead 610, is threaded through the aperture 607 of the tack 601 while the tack 601 is held in the applicator 620. The tack 601 is inserted into a percutaneous access site, locating the target bone or bony structure via fluoroscopy or other imaging method. The lead 610 is positioned as desired for its intended therapeutic purpose. The bone tack 601 is then tapped into place so that the penetrating end 604 sufficiently penetrates the target bone or bony structure and the head crimps the lead. The applicator 620 is then removed.

Thus, the bone tacks 601 of the present invention can be used to secure various devices without the use of sutures. Further, such securing or anchoring can be achieved in percutaneous procedures without the need for a large surgical exposure. And, such securing and anchoring is easily achievable without excessive manipulation, particularly with the use of the deformable head which secures the lead during insertion of the tack into bone. Likewise, this action is assisted by the use of the applicator which is able to hold the tack and deform the head while inserting the tack into the bone.

FIG. 16 illustrates an embodiment of a bone screw 650 of the present invention. The bone screw 650 can also be used to anchor an element, such as a lead or catheter, to a bone or bony structure near to a site of an intended application. The bone screw 650 has a head 652 and a penetrating end 654 opposite the head 652. Typically, the penetrating end 654 has a tapered shank with a helical thread which is suitable for turning or twisting into bone. In some embodiments, the thread is particularly suitable for penetrating cortical bone. Cortical thread forms are generally finer pitched (more threads per inch) and shallower than thread forms designed to penetrate cancellous bone. In some embodiments, the helical thread has a pitch of 0.020-0.200 inches, more particularly 0.029 inches. Typically, the penetrating end 654 is self-tapping and does not require the use of a bone tap to implant the bone screw 650 into the hard tissue. In some embodiments, the penetrating end 654 has an acute nose angle to assist in self-tapping, such as a 60 degree nose angle. In some embodiments, a wedge is added to further assist in self-tapping, such as a 30 degree wedge.

The head 602 includes an aperture 657 through which the element 152 can be threaded prior to implantation of the element 152 in a manner similar to the bone tack 601 of FIG. 4. Or, the screw 650 can be slipped over a portion of the element 152 through a channel 658 in the head 652 which connects to the aperture 657 in a manner similar to the bone tack 601 of FIG. 7. Optionally, the bone screw 650 may include a grommet having similar features to the grommet 606 described previously in relation to bone tacks 601.

FIGS. 17A-17E provide various views of one embodiment of a bone screw 650 of the present invention. FIG. 17A illustrates a perspective view of a bone screw 650 similar to the bone screw of FIG. 16. However in this embodiment, the penetrating end 654 has a thread which is more suitable for penetrating cancellous bone. FIG. 17B illustrates a side view of the bone screw 650 of FIG. 17A. In this embodiment, the head 652 has a diameter of approximately 0.14 inches and an aperture 657 having a diameter of approximately 0.06 inches. Likewise, the head 652 has a 0.03 inch channel 658. The penetrating end 654 has a length of 0.38 inches from the center of the aperture 657 and a diameter of approximately 0.10 inches (as illustrated in the top view of FIG. 17E). Referring to FIG. 17C and its cross-section shown in FIG. 17D, the penetrating end 654 has a shank with a helical thread with a pitch of 0.075 inches. Thus, the bone screws 650 of the present invention typically have a small size to allow positioning in confined or hard to reach areas of the anatomy. It may be appreciated that such dimensions are exemplary and are not intended to limit the scope of the present invention.

The head 652 and a penetrating end 654 of the bone screws 650 are typically formed from the same material and may comprise any biocompatible and/or bioresorbable material including but not limited to cobalt chromium, cobalt chromium alloys, titanium, titanium alloys, stainless steel, resorbable PGA or PLA, and PEEK.

The bone screws 650 of the present invention are driven into a hard tissue, such as a portion of bone B, by rotational force. Referring to FIGS. 18A-18B an applicator 660 is provided for delivery of the bone screw 650 to a portion of a bone B. The applicator 660 is designed similarly to the bone tack applicator 620 in that it has a low profile so that the screw 650 can be delivered through a percutaneous access opening and positioned at an anchoring location via fluoroscopy or other imaging techniques. Again, the applicator 660 typically comprises an elongate body 670 having a proximal end 672 and a distal end 662, wherein the distal end 662 is configured to receive the hard tissue anchor 600. In most embodiments, the applicator 660 also includes a handle attached to the proximal end 672 of the elongate body 670.

FIG. 18A illustrates an embodiment of a distal end 662 of the applicator 660 having a recess 664 for receiving a head 652 of a bone screw 650. The applicator 660 includes a rotatable member 661 which is joinable with the bone screw 650. FIG. 18B illustrates a bone screw 650 securely fixed to the rotatable member 661 via friction, such as with a grommet. The screw 650 is penetrated and anchored into the bone B via rotation of the penetrating end 604 by rotating the member 661. When it is desired to deform or crimp the head 652, force may be applied to the handle and translated to the head 652 which crimps the head 652 onto an element passing through the aperture 657. The screw 650 can then be released from the applicator 660, such as with the use of a release button.

One challenge of a twisting or screw-type penetration is that the orientation of the aperture 657 depends on how the screw 650 is screwed in. Also, placing the lead into the aperture 657 after delivery may be difficult due to its orientation. These challenges are overcome by the bone screws 650 of the present invention. The bone screw 650 may be screwed in place at a desired location first and then the element, such as a lead, is loaded through the channel 658 in the head 652. The lead is then advanced to a desired position for the therapeutic application and secured in place by crimping of the head 652.

Although the foregoing invention has been described in some detail by way of illustration and example, for purposes of clarity of understanding, it will be obvious that various alternatives, modifications and equivalents may be used and the above description should not be taken as limiting in scope of the invention. 

What is claimed is:
 1. A method for anchoring an element within the body, the method comprising: positioning the element so that a portion resides within the epidural space and portion resides outside of the epidural space; driving a penetrating end of a hard tissue anchor into a surface of a vertebrae; and fixing the portion of the element residing outside of the epidural space to the head of the hard tissue anchor so as to substantially maintain the position of the element. 